The project is directed at development of conditions for isolation of cellular revertant of Kirsten virus transformed cells and characterization of their properties. The desired revertants should contain a rescuable wild-type Kirsten virus and produce the characteristic p21 protein as the src gene product. The system used is NIH 3T3 cell line that had been infected two consecutive times with Kirsten virus. Analysis of the DNA of this cell line showed that it contains two copies of the Kirsten genome. Condition for mutagenesis and selection procedure for isolation of revertant had been worked out. The cell revertants that had been isolated are currently being characterized.